The invention relates to a method for increasing the capacity of an existing urea process, the existing urea process comprising a process in which ammonia and carbon dioxide are fed to a reactor (R) at an elevated pressure (P) forming a reaction mixture (M1) comprising ammonium carbamate, ammonia, water and urea. The reaction mixture is then removed from the reactor and fed into a pressure-reduction section (SC) where the pressure is reduced, in either a single stage or in multiple stages, and heat is supplied to separate gaseous ammonia and ammonium carbamate from the reaction mixture to form a liquid mixture (M2). The gaseous ammonia and ammonium carbamate, once separated, are condensed and separated to obtain ammonia (M3') and an aqueous ammoniacal solution of ammonium carbamate (M3) and reused in forming a urea mixture (M2) from which urea is recovered in a urea-recovery section (U).
Such an existing urea process is described in Kirk-Othmer, Encyclopedia of Chemical Engineering, third edition, Volume 23, pages 553-556. In this publication such a process is described as the `Solution-Recycle Process`. Examples of this process according to this publication are the so-called Mitsui-Toatsu Total Recycle C-Improved Process, the Montedison Urea Process and the UTI Heat-Recycle Process. This process for preparing urea is characterized in that ammonia and carbon dioxide are fed directly to a reactor. This process is furthermore characterized in that the excess ammonia and ammonium carbamate are separated from urea by reducing the pressure and simultaneously supplying heat. This in contrast to the so-called High-Pressure Gas-Stripping processes, such as the Stamicarbon CO.sub.2 Stripping Process and the Snamprogretti NH.sub.3 Stripping Process, in which the ammonium carbamate is recovered from the reaction mixture by stripping with ammonia, carbon dioxide or a different gas (at almost the same pressure as the pressure in the urea reactor), after which the gaseous mixture is condensed and returned to the reaction zone.
A method known to a person skilled in the art for substantially increasing the capacity of such `Solution-Recycle Processes` is to replace equipment that constitutes a bottleneck in the process by larger apparatuses. Examples of equipment to be replaced are the urea reactor (R) and the intermediate pressure step (the first step) of the pressure-reduction section (SC). When these apparatuses are replaced by larger apparatuses the expensive high-pressure pumps for ammonia and ammonium carbamate will also often have to be replaced or added.
A drawback of the above method is that it is very expensive. This is partly caused by the high capital investment costs involved in replacing the urea reactor, the equipment for the intermediate pressure step, and the associated high-pressure pumps.